The present invention relates to antennas and, more particularly, to patch antennas.
Patch antennas, which are typically characterized by a flat radiating element placed in close proximity to a ground plane, are used for many beneficial purposes, such as for individual elements in phased array antennas. Such patch antennas are gaining in popularity due, in part, to their relatively small size and relatively low production cost as compared to other types of antennas. The various uses of patch antennas are well known and will not be discussed further herein.
Patch antennas typically consist of a radiating patch separated from a ground plane by a dielectric substrate. Referring to FIG. 1, for example, a patch antenna in a typical prior implementation consists of a ground plane 101, radiating element (patch) 102, conducting probe 103, and standoffs 105, illustratively manufactured from a dielectric material, which are located around the patch's edges to separate the patch 102 from the ground plane 101. Conducting probe 103 is, for example, a conducting Radio Frequency (RF) transmission line such as, for example, an inner conductor of a well-known coaxial cable 104. The inner conductor 103 of conducting probe 103 is connected to patch 102 and is the conduit by which RF signals are passed to the patch 102. In operations of such a patch antenna, electromagnetic signals are input to the patch 102 via inner conductor 103 of coaxial cable 104 causing electrical currents to be induced on both the patch 102 and ground plane 101 and polarization currents to be induced in dielectric substrate 105 all of which in turn radiate electromagnetic wave in free space.
One skilled in the art will recognize that many different structures can be used in the manufacture of the patch antenna of FIG. 1 with various effects. For example, instead of using dielectric standoffs, the patch in some implementations is separated from the ground plane simply by air or a solid substrate of dielectric material. As is well-known, a dielectric material is a material that is a poor conductor of electricity, but one that can efficiently impact on electric field strength and on speed of electromagnetic wave traveling inside volume filled with said dielectric material. The use of such dielectric materials in many applications is extremely well-known. Dielectric materials are typically characterized by a dielectric constant, also called the dielectric permittivity ∈ of the material. The impact of dielectric material on patch antenna performance depends not only on dielectric permittivity ∈ but also on size and shape of substrate. Thus, the effective permittivity ∈eff of the substrate is often used instead of the permittivity ∈. This effective permittivity ∈eff is generally a complicated function of both the permittivity ∈ of the substrate material as well as the size and shape of the substrate. The first order approximation of the effective permittivity ∈eff is directly proportional to ∈. As is well-known, the length l of an antenna patch necessary to operate at a given frequency f is a function of the ∈eff of the substrate. Specifically, the length l can be defined by the following equation:
                    l        =                  c                                    f              ⁡                              (                                  ɛ                  eff                                )                                                    1              /              2                                                          (                  Equation          ⁢                                          ⁢          1                )            where c is the well-known constant value for the speed of light. In order to achieve the smallest possible length of the antenna patch it is desirable to use an appropriate substrate having the highest ∈eff value.
The operating characteristics of patch antennas, such as the patch antenna of FIG. 1, may be varied depending upon the physical dimensions and materials used in constructing the antenna. For example, as discussed above, for a given operating frequency, the size of the antenna must increase if a dielectric material with a lower dielectric constant is used. For this reason, air is sometimes used as a dielectric material since the ∈eff of air is 1.0. Similarly, the length and/or width of the patch of an antenna may be increased to produce a lower operating frequency (also referred to herein as the resonant frequency). Also, the larger the antenna size, the narrower the antenna angular response pattern, which is the power flux produced by the antenna as a function of the angle relative to the center axis of the antenna. Additionally, all else equal, the operating frequency bandwidth of a patch antenna is influenced by substrate thickness. One skilled in the art will recognize how such dimensions will increase or decrease the resonant frequency and other operating characteristics of the antenna as a result of varying the dimensions of different components of the patch antenna. For example, patch antennas, such as the patch antenna of FIG. 1, are typically characterized by a relatively small operating frequency bandwidth due to the proximity of the patch to the ground plane in such antennas. Illustratively, the distance between the patch and the ground plane is approximately 1/20 of wavelength of signal to be transmitted or received by the antenna. As is well understood, increasing the thickness of a given substrate will desirably result in a corresponding increase of operating frequency bandwidth. However, such an increase in thickness will also undesirably increase the weight of the antenna.